The silver oxide battery is a voltaic cell that uses silver oxide (Ag2O) as the positive electrode material (cathode active material) and zinc as the negative electrode active material (anode active material). Owing to its ability to maintain a constant voltage over a prolonged period, this battery is used mainly as a power source for driving quartz-oscillator watches and clocks, the integrated circuitry of game machines, and the like.
The silver oxide battery is generally structured as follows. The silver oxide powder serving as the active material of the battery positive electrode is blended with at least one member selected from among MnO2, NiOOH, CoOOH, AgNiO2, AgCoO2, CaO, MnO, HgO, CdO, CdS, polytetrafluoroethylene, metallic silver, AgO and carbon. The blend, called the “positive electrode compound,” is generally formed into a circular shape (the “compacted compound body”) using a die press.
The compacted compound body is installed in a can (positive electrode can) made of stainless steel or a stainless steel laminate, a separator is mounted on the compacted compound on the open side of the can, another can is charged with negative electrode zinc paste (negative electrode can), and the positive and negative electrode cans are joined. A nylon ring is generally used as the insulating sealing material interposed between the positive and negative electrode cans. NaOH, KOH or a mixture thereof is used as the electrolyte. The electrolyte is usually injected after the compacted compound body is installed in the positive electrode can. Electrolyte is also sometimes added to the negative electrode zinc paste. The assembled battery is pressure-cured to facilitate permeation of the electrolyte into the compacted compound body.
Among the properties required of a silver oxide battery, the most important is considered to be long service life, for example, the ability to hold up under use for five or more years. The battery is required to possess properties that do not change even under high-temperatures or during storage at normal temperature for several years. In actuality, however, silver oxide (Ag2O) is unstable in an electrolyte. As a result, cases may arise in which self-discharge occurs because the Ag2O dissolves in an alkaline solution and the dissolved Ag ions reach the Zn negative electrode or because Ag is precipitated owing to a decomposition reaction of the Ag2O itself.
A technique has been developed for preventing such self-discharge by interposing cellophane tape between the positive electrode and negative electrode so that dissolved Ag ions are caught by the cellophane and prevented from dispersing to the negative electrode. Another practice adopted is to further install a polypropylene or PEGF film between the positive electrode and the cellophane, to establish a multilayer arrangement. Decline in the function of the cellophane owing to oxidation by the Ag ions is, however, unavoidable. Moreover, the extent to which the separator can be formed in multiple layers is limited by the limitation on the volume of the battery.
In light of this situation, positive electrode side solutions proposed include that taught by JP Sho59-167963A of adding Cd to the positive electrode compound so as to curb dissolution of silver, that taught by JP Sho55-133765A of adding zinc oxide to the Ag2O, and that taught by JP Hei2-12762A of sandwiching the silver oxide positive electrode with a shaped body of mixed manganese dioxide and carbon.